Method of fabrication of electrical heater



Nov. 7, 1967 'c. MENDELSOHN 3",35o,777

METHOD OF. FABRICATION OF ELECTRICAL HEATER Original Filed Jam. '7, 1963ii w 9 F1614 G- 'G' v 1 WRLES MESZSFZZS;..

ATTORNEY United States Patent 3,350,777 METHOD OF FABRICATION OFELECTRICAL HEATER Charles Mendelsohn, Monsey, N.Y., assignor to General1 This invention relates to electrical resistance heating devices andmethods of their manufacture.

This application is a division of application Ser. No. 249,624, filedJan. 7, 1963 in the name of Charles Mendelsohn, which applicationwas'assigned to the same assignee as the present invention and is nowabandoned.

Electrical resistance heating and a vast number of heating devices ofdiverse types which employ this principle are widely known in the art.These devices are used in an almost countless number of applications andit would seem that a size and type suitable to any need must beavailable. Such is not the case, however. a

In the gyroscopes and stable platforms used in inertial guidancesystems, both space and weight are at a very high premium and there haslong existed a great need for heating elements of extremely smallthickness dimensions (e.g., 0.005 to 0.008") and a power density in theorder of at least 40-60 watts per square inch.

Heretofore, heating of gyros has been accomplished by means ofhand-assembled arrangements consisting of Nichrome wires embeddedbetween plastic sheets or wrapped around the gyro and encapsulated withepoxy.

The disadvantages .of these prior art heating arrangements, aside fromthe high cost engendered by their hand assembly, stem from the fact thatthe completed units are quite thick; consequently, they cannot beinstalled for optimum heating of the platform. This, in turn, may leadto larger-than-necessary power consumption and/ or insufficient orun-uniform heating.

The fundamental object of the present invention is to overcome ormitigate at least one of the problems extant in the prior art asoutlined above.

A more particular object is the provision of electrical resistanceheater elements which are extremely thin, flexible, durable and have arelatively high power capacity.

Another object is the provision of electrical resistance heater elementswhich are particularly, if not exclusively, adapted for use in heatinggyros, stable platforms, and the like. a

A further object is the provision of electrical resistance heaterelements which do not require manual assembly and are, therefore,lowerin cost than comparable heater elements known heretofore.

A still further object is the provision of methods for fabrication ofheater elements characterized as in the preceding objects.

To the fulfillment of these and further objects, the present inventioncontemplates electrical resistance heaters comprising a laminatedassembly of at least three flexible film-like layers arranged and bondedtogetherin congruent superposition. At least the inner surfaces of theouter layers are electrically non-conductive, the intermediate layerbeing composed of at least one continuous elongated strip of electricalresistance heating material. Electrical contacts to spaced points on thestrip, accessible through theouter layers, are provided.

Briefly stated, the method of fabrication contemplated by the inventioncomprises: providing a first film at least one surface of which iselectrically non-conductive; forming on the one surface a film ofelectrical resistance heating material in the form of at least onenarrow continuous 3,350,777 Patented Nov. 7, 1 967 ICC strip ,havingvelectrical contacts at spaced points thereon; and applying a second filmof flexible material, having at least one electrically non-conductivesurface, over said strip and bounding said first and second filmstogether with the respective non-conductive surfaces thereof inconfronting relation.

Additional objects of the invention, its advantages, scope and themanner in which it may be practised will be more fully apparent topersons conversant with the art from the following description ofexemplary embodiments thereof taken in conjunction with the subjoinedclaims and the annexed drawings in which like parts are designated bylike reference numerals throughout the several views and FIGURE 1 is aplan view of a heater element according to the present invention;

FIGURE 2 is a sectional View on line 2-2 of FIGURE 1 looking in thedirection of the arrows;

FIGURE 3 is an enlargement of a portion (the lower end) of FIGURE'2;

FIGURE 4, 5 and 6 are views similar to FIGURE. 1, 2 and 3, respectively,showing a modifiedembodiment of the invention.

Referring now to the drawings and, first, particularly to FIGURES 1, 2and 3, there is illustrated a heater element 10 consisting of three thinflexible layers 12, 14 and 16 laminated in congruent superposition.

While element 10 is shown in FIGURE 1 as being square in planconfiguration it will be understood that the variety of shapes anddimensions which may be used is limited only by the needs of theparticular application and the feasibility of manufacture. On the otherhand the overall thickness dimension T of element 10 is in all casesextremely small, i.e., in the order of a few thousandths of an inch. Itis well apparent, therefore, that the sectional view of FIGURE 2 is agross exaggeration of the thickness of the element necessary to permitsatisfactory illustration.

In one practical embodiment of element 10 the total thickness dimensionwas .005 inch, which was comprised of two outer layers, 12 and 14, each.002 inch in thickness and intermediate layer 16, .001 inch in thick-'ness.

In the embodiment illustrated in FIGURES l, 2 and 3, outer layers 12 and14 consist of aluminum foil at least the inner surfaces 12a and 14a,respectively, of which are hard anodized to provide an electricalinsulation layer a which is not separately shown in the drawings.

strip of resistance heating alloy foil preferably of the Theintermediate layer 16 is a highly convoluted thin nickel-chromium alloyvariety, e.g., Nichrome.

One of the outer laminations, foil layer 14 in the illustratedembodiment, contains a pair of clearance holes 18,

20 suitably located to register with contact padenlargemerits 22, 24 atthe respective ends of strip 16, thus providing access to theseenlargements for making electrical connections thereto. It will beunderstood that while a single, uniformly-distributed,regularly-convoluted strip 16 has been shownrand described in theinterests of simplicity, a plurality of such strips, intertwined,interdig-itated, or strips having convolutions confined to orconcentrated in selected regions, can be employed to regulate and/ orvary the pattern of heating or other heat flow parameters.

In such an event at least one additional access opening would beprovided for each additional strip (assuming the use of one Opening fora common ground or return connection).

If desired, the laminations may be preformed tofit a particularconfiguration of the part to be heated.

The method contemplated by the invention for fabrication of element 10is as follows. A sheet of aluminum foil is provided with a hard anodizedcoating on at least one of its major surfaces. To the anodized surfaceis bonded, by means of a high temperature epoxy, a sheet of Nichromefoil. An epoxy adhesive such as that commercially available under thedesignation Bondmaster M-688 is well suited to this purpose.

The desired convoluted strip pattern of layer 16 is then defined on theexposed surface of the Nichrome foil in the form of an etch-resistantcoating selectively applied by masking or photoresist techniques wellknown in the art and widely used in the fabrication of semiconductordevices, for example. The Nichrome foil surface is then exposed to anetching agent which attacks the areas which are not covered by theresistant coating. After the exposed Nichrome areas are completelyetched away, the resistant coating may be removed leaving the desiredconvoluted patern constituting layer 16.

It will be understood that if the etching step involves contact of thealuminum foil with the etchant, this also should be covered with asuitable protective coating, e.g., wax which is subsequently removed.

A second sheet of aluminum foil, having a hard anodize coating on atleast one surface and suitable access openings such as 18, 20 isprovided; to complete the element this sheet is placed over the Nichromepattern with its anodized surface confronting that of the first aluminumsheet and with the access openings in registration with contact padenlargements 26, 22 and cemented in position with a suitable epoxyadhesive or the like.

While epoxy adhesives have been specified for the bonding operations itwill be understood that other suitable materials may be employed. As ageneral rule high temperature epoxies give satisfactory results up toabout 200 C.; where higher temperatures are encountered, ceramic-typeadhesives (e.g., Corning Glass Companys Pyro-Ceram) should be used.

In use, the heating element is placed with one surface in contact withthe part to be heated; due to the high thermal conductivity of aluminum,heat transfer from the Nichrome layer to the part to be heated isaccomplished with high efficiency. Moreover, the aluminum layer on theopposite side of the Nichrome layer with respect to the member to beheated, functions as a reflector further improving heating efficiency.If desired, a black anodize coating can be applied to the appropriateexterior aluminum surface to enhance its emissivity.

Because the element is so thin and flexible it is susceptible of beingwrapped closely around parts to be heated, thus enabling theestablishment of intimate surface contact between the two withconcomitant high efiiciency in heat transfer. A heating element of thetype just described has a power handling capacity in excess of 200 wattsper square inch.

A modified form of heating element is shown in FIGURES 4, 5, 6. Element10 is similar, and may be identical, in structure to that shown inFIGURES 1-3; however, the outer laminations 12' and 14" consist ofepoxy-cementable polytetrafluoroethylene films (such as the Du PontCompanys PEP-C Teflon). The convoluted resistance heater strip 16 may beformed in the manner already described and the laminations bondedtogether With high temperature epoxy adhesive.

If desired, a fourth layer, not shown, consisting of a thin film (in theorder of 0.001 inch thick) of aluminum 5 may be applied to the exteriorsurface of one of the outer layers 12, 14 to serve as a heat reflector.

An alternative form of construction contemplated by the invention is anelement in which one of the outer layers is hard-anodized aluminum foilas in the FIGURES 1-3 embodiment and the other is cementablepolytetrafluoroethylene as in FIGURES 46.

Heating elements incorporating Teflon lamin-ations in accordance withthe foregoing description are capable of up to 75 watts per square inch.

While there have been described what at present are believed to be thepreferred embodiments of this invention it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is aimed,therefore, to cover in the appended claims all such changes andmodifications as fall within the true spirit and scope of the invention.For example, the convoluted resistance heating strip may be applied byvapor deposition of Nichrome or the like through an appropriatelyapertured mask.

What is claimed is:

1. A method of fabricating an electrical resistance heater comprising:

providing a sheet of aluminum foil hard anodized on at least one majorsurface;

bonding a sheet of Nichrome foil to the anodized surface of the aluminumfoil; chemically etching away selected regions of said Nichrome foilsheet to form a pattern made up of at least one continuous,highly-convoluted narrow strip of Nichrome traversing substantially theentire anodized surface of said aluminum foil; and

bonding a sheet of material having at least one electricallynon-conductive surface to said aluminum foil with said non-conductivesurface confronting the anodized surface of the aluminum foil andcovering said Nichrome strip.

2. The method of claim 1 where said sheet of material is a film oftetrafluoroethylene polymer.

3. The method of claim 1 where said sheet of material is aluminum foilhaving at least said one surface thereof hard anodized.

References Cited UNITED STATES PATENTS JOHN F. CAMPBELL, PrimaryExaminer.

J. M. ROMANCHIK, I. CLIN-E, Assistant Examiners.

1. A METHOD OF FABRICATING AN ELECTRICAL RESISTANCE HEATER COMPRISING:PROVIDING A SHEET OF ALUMINUM FOIL HARD ANODIZED ON AT LEAST ONE MAJORSURFACE; BONDING A SHEET OF NICHROME FOIL TO THE ANODIZED SURFACE OF THEALUMINUM FOIL; CHEMICALLY ETCHING AWAY SELECTED REGIONS OF SAID NICHROMEFOIL SHEET TO FORM A PATTERN MADE UP OF AT LEAST ONE CONTINUOUS,HIGHLY-CONVOLUTED NARROW STRIP OF NICHROME TRAVERSING SUBSTANTIALLY THEENTIRE ANODIZED SURFACE OF SAID ALUMINUM FOIL; AND BONDING A SHEET OFMATERIAL HAVING AT LEAST ONE ELECTRICALLY NON-CONDUCTIVE SURFACE TO SAIDALUMINUM FOIL